The press-hardening of steel is a technique that has been known since the 1970s. In this method, a steel sheet blank with an alloy composition that is matched to the press-hardening method is heated to a temperature that permits an austenitization and preferably a complete austenitization. A complete austenitization usually occurs above the so-called AC3 point, which can be read from corresponding multiple-substance phase diagrams and in particular, also depends on the composition.
After the heating and complete austenitization, such a steel is cooled at a speed that lies above the so-called critical hardening speed. This produces a fully martensitic structure, which gives the steel a high hardness, in particular of up to 1500 MPa and above.
The hardness of such a press-hardened steel is essentially determined by the carbon content since this determines the martensite hardness.
Other alloy elements in the composition essentially cooperate with the carbon to determine the hardenability; certain elements including boron influence the transformation behavior and in particular, function as so-called transformation-delaying elements. These transformation-delaying elements significantly decrease the temperature below which—even with the cooling above the critical hardening speed—fully martensitic structure would no longer be achievable and can therefore in some circumstances be used to favorably influence certain process parameters.
The usual procedure in press-hardening is to provide the corresponding steel, which is to be press-hardened, in the form of a sheet, to cut a sheet blank from this sheet, and to either deep-draw this sheet blank in a cold state and then heat it, insert it into a tool, and correspondingly cool it by means of contact with the cooling tool on all sides, or to heat the sheet blank and hot-form it in a tool and at the same time, to cool it at the corresponding speed.
In this intrinsically known method, the cooling rates are determined by the tool or more specifically by the contact of the press-hardening steel with the tool. In this connection, a low thermal conductivity, a low heat capacity, the heat transfer, the pressing pressure, and the percentage of press area, but also the flow temperature of a cooling medium such as water can influence and in particular reduce the achievable cooling rates.
In practice, it has also turned out that in the press-hardening method, due to the transfer of the hot sheet from the furnace to the press and in particular also due to high emissivities (high thermal radiation behavior) of the sheet or sheet blank, undesirable diffusion-controlled transformations can occur at high temperatures (ferrite).
It has also been possible to determine that the deep-drawing of these sheets in the hot state accelerates the transformation so that in this case, ferrite and bainite will form before the martensite does.
The object of the invention is to disclose a method for press-hardening steels, which facilitates and improves process control during press-hardening and makes it more reproducible.